Conventionally, as a material for an aluminum heat exchanger, there has been used various aluminum materials having a brazing material and a sacrificial material (hereinafter, frequently referred to as “surface material”) on one side or both sides of the aluminum material. Especially, as a material for a heat exchanger for an automobile, it is demanded that the aluminum material be made thinner while high strength and high corrosion resistance are maintained. In order to meet such demands, for example, JP-A-2002-294377 discloses an aluminum alloy composite material for brazing having high strength and high corrosion resistance, which is made notably thin while high brazing property is retained.
In the aluminum alloy composite material for brazing disclosed in the above-mentioned patent document, an aluminum alloy plate includes a core material and a Zn, Mn, Si-containing surface material cladded on one side of the core material, while a composition of the core material, a composition of the surface material and a thickness thereof are adjusted.
Specifically, in the patent document above, the core material composition includes 0.2% by mass or less of Mg, 0.3% by mass or less of Cr, 0.2% by mass or less of Fe, 0.2-1.0% by mass of Cu, 0.3-1.3% by mass of Si (with a total amount of Cu and Si being 2.0% by mass or less), 1.5% by mass or less of Mn, 0.02-0.3% by mass of Ti. The rest includes Al and inevitably included impurities.
The surface material composition includes at least one member selected from 2-5% by mass of Zn, 0.3-1.2% by mass of Mn, and 0.04-0.9% by mass of Si, and the rest includes Al and inevitably included impurities. The clad ratio of the surface material is set at 15% or more of the total thickness. In this manner, the aluminum alloy composite material is optimized.
However, more corrosion resistance is demanded for the surface material, in order to enhance the endurance of the heat exchanger. When the aluminum alloy composite material with the Mn, Si-containing surface material, such as those described in the above document, is brought into contact with water, a water-containing coolant or the like, a cathode reactivity increases at an interface therebetween, leading to a problem of increased cathode reaction current upon corrosion progress, i.e., increased corrosion current. Therefore, once a local corrosion, such as pitting corrosion, occurs on the surface material, a corrosion rate is locally accelerated. Therefore, the optimization of the composition of Zn, Mn, Si of the surface material and the optimization of the clad ratio of the surface material may not provide sufficient corrosion resistance.
The present invention is completed with a view toward solving the above-mentioned problems. It is desirable to provide an aluminum alloy plate having high strength and excellent corrosion resistance even though the plate is made thinner, and a heat exchanger having excellent corrosion resistance formed of this aluminum alloy plate.